Computer programs and data, such as computer program source code or a markup language definition of a web page, can be interpreted, translated, or compiled by a compiler to produce instructions more readily executable by a computer. These actions are often performed with reference to external program code or markup language definitions that are used as building blocks for the computer program or markup language definition. The compiler, external program code, and markup language definition building blocks can be provided by a software vendor or service provider, who may upgrade, i.e., change or enhance, the building blocks, and provide new versions to be used in place of previous versions.
With some conventional approaches, “client” computer programs access the features of the building blocks through defined interfaces, e.g., particular function names with particular parameters, or particular markup language definition names. The interfaces are meant to allow the provider to change and enhance the details of the building block implementations without affecting the interfaces used by the client programs. Thus, the building blocks are upgraded from time to time by replacing an existing file of external program code or markup language definitions with a new version of the file. The new version may contain the same interfaces, in which case the client program interactions with the new version are expected to remain the same as in the previous version, and the features of the new version are ordinarily expected to be substantially compatible with those of the previous version. In some cases, however, the client program does not continue to work properly with the new version. For example, unexpected problems may occur if there are errors in the new version or ambiguities or misunderstandings in the client's use of the interfaces.